An infrared detector of the prior art is, as shown in FIG. 15, provided with a pyroelectric element 100 for detecting infrared rays radiated from a human body, a current-to-voltage converting circuit 200 for converting a detection current signal of the pyroelectric element 100 into a voltage signal, a coupling capacitor C30 connected with the current-to-voltage converting circuit 200, a voltage amplifying circuit 300 connected with an output of the coupling capacitor C30, a low-pass filter 400 connected with the voltage amplifying circuit 300, a high-pass filter 500 connected with the low-pass filter 400, an amplifying circuit 600 connected with the high-pass filter 500, and an output circuit 700 connected with the amplifying circuit 600.
The current-to-voltage converting circuit 200 includes a FET (Field Effect Transistor) having a gate connected with the pyroelectric element 100, a resistor Rg connected in parallel with the opposite ends of the pyroelectric element 100, and a resistor Rs provided between a source of the FET and a ground. The low-pass filter 400 and the high-pass filter 500 each include a switched capacitor.
The infrared detector thus constructed operates as follows. A detection current signal outputted from the pyroelectric element 100 is converted into a voltage signal by the resistor Rg and applied to the gate of the FET, whereby a drain current flows from the source of the FET to a drain thereof. A source voltage is generated between the FET and the resistor Rs by the flow of the drain current. After direct-current (dc) components of the source voltage is cut by the coupling capacitor C30, and the source voltage is amplified at an amplification factor (1+R20/R10) by the voltage amplifying circuit 300, and is consequently handled as a voltage signal in a specified frequency band whose high-frequency components and low-frequency components have been cut by the low-pass filter 400 and the high-pass filter 500. This voltage signal is amplified at a set gain by the amplifying circuit 600, and is outputted as a detection signal from the output circuit 700 after being compared with a specified level therein.
However, the above coupling capacitor C30 needs to have a large capacity in order to pass frequency components around 1 Hz expressing the motion of the human body, etc. Thus, a large-size capacitor has to be used as such. The large-size capacitor is obliged to be separately externally provided since it is difficult to integrate. Such an externally provided coupling capacitor C30 has been standing as a large hindrance to miniaturization or integration of the infrared detector.